Rolling mills of this kind comprise a plurality of side-by-side stands whose rolls define a rolling path. Along this rolling path, a tube is stretched as a consequence of the different rotational speeds of the rolls, which increase progressively from the input stand to output stand.
On the above rolling mills, therefore, each stand must be driven at a defined speed, that is, the mill must be operated to a "speed profile", meaning the aggregate of the speeds of the various stands when plotted as a graph with the sequential order numbers of the stands on the abscissa and the speed of each stand ont he rodinate, is described by a point set lying on a curve.
In order to perform different rolling operations, as well as to maintain control of any rolling operation, it is mandatory that the speed profile be changeable.
Several ways of changing the stand speeds have been known. The conceptually most straghtforward approach is that of providing each stand with a single independent motor of its own. Thus, maximum flexibility and adjustability of the speed curve can be achieved. However, in view of the large number of stands supplied on rolling mills of this type (up to 20 or 30 stands), the system is difficult to manage due to the large number of degrees of freedom; this enforces the availability of highly complex computers and extremely sophisticated sofware.
To overcome these drawbacks, the so-called differential drives or group drives have been developed.
A first of these nsee German Patent No. DAS-1054408) provides a primary motor and an auxiliary motor, drivingly connected to all the stands through respective differential transmissions; each stand operates at a speed resulting from the combined speedds from the primary motor and the auxiliary motor, each as suitably stepped down. The primary motor will impart a basic speed profile, and the auxiliary motor a corrective profile of the basic profile.
A second type of groups drives (see U.S. Pat. No. 4,388,819) provides a primary motor and two auxilieary motors, drivingly connected to the stands through respective differential transmissions; the primary motor is connected to all the stands, whereas each auxiliary motor is connected to a respective group of stands. Consequently, the primary motor will impart a basic speed profile, and the auxiliary motors corrective profiles of the basic speed profile, each for its respective group of stands.
A third type of drive by groups (see U.S. Pat. No. 4,768,370) provides for the stands to be split into several groups of adjacent stands; the bordering stands between groups are driven directly by respective motors, whereas the intermediate stands are linked to both of the nearest bordering stands through respective differential transmissions. The intermediate stands run at speeds which depend on those of the nearest bordering stands; when the speeds of the bordering stands are varied, the speeds of the intermediate stands also vary accordingly.
The groups drives just outlined hereinabove constitute a compromise between a drive by single motors and a fully rigid drive, i.e. a drive wherein the drive ration of each stand and the single motor is constant.
The flexibility of these group drives is highest with the third type and lowest with the first type; conversely, simplicity of construction and convenience of practical management are highest with the first type and lowest with the third type. In general, it may be concluded that the choise from the various types of viable drives is a typical choice by compromise: if preference is to be given to certain features, other features must inevitably be given up.